Basic literature on operational amplifiers

An op amp (abbreviated as op amp) is a very high gain circuit. In real circuits, some functional module is usually combined with a feedback circuit. This is an amplifier with a special coupling circuit and feedback. Its output can be result of mathematical operations such as addition, subtraction, or differentiation and integration of input signal.

Because in early days it was used in analog computers to perform mathematical operations, it was called an "op-amp".

An operational amplifier is a circuit device, named from a functional point of view, that can be implemented as discrete devices or semiconductor chips.

With development of semiconductor technology, most operational amplifiers exist as a single chip. There are many types of operational amplifiers that are widely used in electronics industry.

One,OpAmp Development History

1941: Carl D. Schwarzel, Jr. of Bell Laboratories invented first vacuum tube op amp and received U.S. Patent 2,401,779 titled "Summing Amplifier";

1952 The first commercial product sold as a commercial product was a tube op amp from George A. Philbrick Researches (GAP/R), model K2-W;

1963: The first single integrated circuit op amp was μA702 designed by Bob Widlar of Fairchild Semiconductors, and μA709 was released in 1965 after modification;

1968: μA741 from Fairchild Semiconductor. It is still used in production to this day, it is most successful op-amp of all time and one of very few IC models with longest lifespan.

Second, basic knowledge of op amps

The op amp has two input terminals and one output terminal as shown in figure below, input terminal marked with "+" is "non-inverting input terminal" and cannot be called positive terminal), other input terminal labeled "one" is "inverting input terminal" and cannot be called a negative terminal either. If same signal comes from these two input terminals in series, output terminal will receive an output signal with same voltage but with opposite polarity: output signal at output terminal is in phase with signal at non-inverting input terminal, and inverting terminal is in phase with signal at inverting input terminal.

The power supply connected to op-amp can be single or dual, as shown in figure below.

Op-amps have some very interesting characteristics that can be used flexibly for many unique purposes. In general, these characteristics can be combined into two:

1. The increase of operational amplifier is infinite.

2. The input impedance of an op amp is infinite and output impedance is zero.

Now let's take a quick look at what conclusions can be drawn from above two properties.

First, gain of an op-amp is infinite, so as long as input voltage of its input terminal is non-zero, output terminal will have an output voltage equal to positive or negative power supply. output voltage must be infinitely high, but limited by supply voltage.

For sure, if input voltage at non-inverting input is higher than input voltage at inverting input, even if it's only slightly higher, op-amp's output will produce same voltage as input. positive supply voltage;< /p>

Conversely, if input voltage of inverting input is higher than input voltage of non-inverting input, op-amp will output a voltage equal to negative supply voltage (if op-amp uses a single power supply, then output voltage is zero).

Second, because gain is infinite, op amp cannot be used directly as an amplifier, and output must be fed back to an inverting input (called negative feedback) to reduce its gain.

As shown in figure on left in Figure 1-3, function of resistor R1 is to return output signal to inverting input of op-amp. Since voltages at inverting input and output are opposite, circuit will be reduced. The increase is a negative feedback circuit, and Rf resistor is also called a negative feedback resistor.

Also, since input of op-amp is infinite, no current is applied to input of op-amp - it only accepts voltage.

Similarly, if you imagine that there is an infinite resistor between non-inverting input and inverting input of op-amp, then voltage applied across resistor cannot produce a current. There is no current. According to Ohm's Law. There will be no voltage across resistor, so we can assume that voltage across two input terminals of op-amp is same. .